Thixotropic cement slurries have been used for many years in cementing conduits into drilled bore holes to prevent fluid loss from the well to the surrounding earthen formation. The prior art suggests a number of compositions mixable with water to form a slurry capable of reducing the fluid loss to the surrounding formation or capable of providing a hardened cement to permanently set the conduit into position without fluid loss. Among these prior art compositions, it is known to use bentonite clay in pellet or slurry form to prevent fluid loss and it is known to use a mixture of bentonite clay with Portland cement settable to a hard condition for permanently fixing the conduit within the bore hole. The following patents disclose various soil sealing compositions:
Mason U.S. Pat. No. 4,463,808 discloses a well fluid and bore hole sealing composition including water, a water-swellable clay and a water-dispersible polymer, such as an emulsion polymerized hydrolyzed polyacrylamide to prevent the immediate hydration of the clay.
Tazawa et al U.S. Pat. No. 4,004,428 is directed to a process for stabilizing soil by injecting into the soil a grout mixture comprising an aqueous sodium silicate solution, an aqueous solution of a gelling agent consisting of chlorides, sulfates and nitrates of aluminum, magnesium and iron and then gelling the injected mixture in the soil by adding at least one oxy acid.
The Kim et al U.S. Pat. No. 3,615,780 is directed to the gelling of an aqueous solution of vegetable polyphenolic material derived from coniferous tree bark or tannins of catechin or condensed type by reaction with alkali metal silicates, preferably sodium or potassium silicate. The compositions may be modified with additional material such as bentonite clay, Portland cement and the like.
The Chesney, Jr. et al U.S. Pat. No. 4,447,267 is directed to a Portland cement based grout composition including bentonite clay, a filler and a composite of a cellulose ether and a long chain polymer, such as polyacrylamide.
Crinkelmeyer et al U.S. Pat. No. 4,102,400 is directed to a composition for contacting a well bore prior to disposing a thixotropic cement slurry in the well bore to initially prepare the well bore to prevent fluid loss. The slurry, used prior to the cement, is a gel formed by the reaction of a silicate with a multivalent metal cation; an inert particulate filler and a water-dispersible cellulose compound. This fluid is referred to as a "spacer fluid" since the fluid is frequently used ahead of the cement to displace the drilling mud from the bore hole.
Cementing or grouting of steel well casings in well bores dates back to the early twentieth century. Early uses were directed to filling a cement grout in the well bore annulus surrounding the casing to separate water from oil producing zones and for the completion of water wells. Wells are cased and cemented down to at least a lowest possible pumping level to prevent contamination of the pumped fluid from the surrounding earthen formation. In the case of impervious strata above the oil or water producing gravel or sand, the cement should extend from the surface down to the impervious strata. Further, more and more attention is now given to proper plugging of abandoned wells for the purpose of environmental protection. Cementing or grouting of abandoned wells is an environmentally acceptable procedure. The cement or grout cements the annular space between a hole and a liner or casing, and enters cavities in the surrounding earthen formation to seal the earthen formation against fluids entering the annular space.
The most typical cement compositions used in preventing well bore contamination and for plugging abandoned wells include bentonite clay pellets; neat cement compositions and compositions including both Portland cement and bentonite clay. Bentonite clay pellets, while generally effective in providing a water-impermeable layer surrounding the well casing, are difficult to position within the well bore annulus surrounding the well casing, particularly in small annular spaces. Bentonite pellets are dropped within the annular space and fall randomly creating various sized void spaces between pellets at different locations. The pellets sometimes become sticky and are difficult to drop within small annular spaces, but, if properly positioned, are effective because of the high concentration of water-swellable bentonite, such as sodium bentonite. Generally, although the pellets themselves are essentially 100% bentonite, spacing between pellets creates effectively about a 50% to 60% bentonite concentration within the annular space. The pellets must be positioned in their intended location prior to complete hydration so that, once positioned, the clay can swell to eliminate the void spaces. Consequently, bentonite pellets have a maximum useable depth through water of about 500 feet. The composition of the present invention has no initial void spaces and can displace water in a well annulus to depths of more than 1000 feet.
Water-swellable bentonite clay, when used together with Portland cement, aids in reducing shrinkage of the cement or grout composition but, still, substantial shrinkage occurs, sometimes resulting in undesirable inter-aquifer transfer. Other problems encountered with the above-described prior art cement compositions include a high alkalinity which can alter geotech analyses that rely on accurate pH determination for detection of metal ion contamination; permeability of the cement or grout composition after setting because of the properties of the composition or because of cracking of the cement due to ground shifting; the expense of Portland cement; heating of the cement or grout during curing causing weakening of the well casing, particularly where polymeric casings are used; corrosion of the iron containing well casings because of toxicity of the cement or grout composition, or due to inadequate filling, e.g. shrinkage or cracking of the composition within the annulus, resulting in contamination of the recovered fluid or inadequate well plugging; abrasiveness of the cement or grout on the mixing equipment; initial relatively high viscosity of the composition when mixed with water resulting in more difficulty in completely filling an annulus, with bridging sometimes occurring in the annulus causing inter-zone transfer of fluid and/or contamination; and a non-flexible set cement resulting in cracking upon ground shifting or shrinkage and fluid contamination. The composition of the present invention solves or improves each of the deficiencies in the above-described prior art compositions.